Transformer coil having improved short circuit strength



April 18, 1967 R. w. RUSSELL TRANSFORMER COIL HAVING IMPROVED SHORTCIRCUIT STRENGTH Filed Dec. 31, 1964 INVENTOR. Robert ZflRz/ssel BYUnited States Patent 3,315,197 TRANSFORMER COIL HAVING IMPROVED SHORTCIRCUIT STRENGTH Robert W. Russell, New Concord, Ohio, assignor toMcGraw-Edison Company, Milwaukee, Wis., a corporation of Delaware FiledDec. 31, 1964, Ser. No. 422,793

4 Claims. (Cl. 336-84) This invention relates to coils for high voltageelectrical transformers and, in particular, to such coils having taps ineither winding.

The current in the secondary winding of an electrical transformer flowsin a direction around the magnetic core opposite to the direction of thecurrent in the primary winding, and the two windings therefore repeleach other when the transformer is loaded. The mechanical forces tendingto separate the two windings become very high during severe overloads orshort circuits on the transformer. Taps in either winding will displacethe magnetic centers of the two windings and unbalance the magnetic fluxdistribution so that the leakage flux has a substantial radialcomponent, and consequently a large axial force exists tending todisplace the two windings relative to each other in an axial direction.

It is an object of the invention to provide a transformer coilconstruction having taps in one of the windings which can withstandsubstantially greater short circuit current without deformation thanprior art coils.

It is a further object of the invention to provide a distributiontransformer coil having novel shielding means which permits it towithstand, without deformation, higher magnitude short circuit currentthan prior art coils.

These and other objects of the invention will be more readily apparentfrom the following detailed description when taken in conjunction withthe accompanying drawing wherein:

FIG. 1 is a partial cross sectional view through a core type transformerembodying the invention; and

FIG. 2 is a schematic cross sectionalview through a coil of a shell typetransformer embodying the invention.

Referring to FIG. lof the drawing, a core type distribution transformermay include a generally rectangular magnetic core of fiatwise bentmagnetic strip steel laminations 11 and havinga pair of straight windinglegs 12 (only one of which is shown in the drawing) each of which issurrounded by a coil 14 comprising a secondary winding section 15 and aprimary winding section 16 radially outward therefrom. Alternatively theprimary winding section can be radially inward from the secondarywinding section. One of the winding sections 15 or 16 in each of thecoils 14 is provided with taps to permit changing the turn ratio of thetransformer, and as shown in the drawing the taps are in the primaryWinding section 16. The secondary winding section 15 may be ofconventional construction and comprise a number of concentric layers ofconductor tu rns such as helical turns of wire or turns of conductivestrip wound on an insulating support sleeve 18 surrounding core windingleg 12. An annular insulating barrier 19 is disposed between thesecondary winding section 15 and the primary winding section 16.

Primary winding section 16 may comprise a plurality of concentric layers20, 21, 22, 23 and 24 of helical turns of wire 28 surrounding insulatingbarrier 19 with interlayer sheet insulation 29 disposed between adjacentlayers of wire turns. Tap leads T1 and T2 may be connected to taps inthe radially inward layer adjacent the start of primary winding section16.

Tapping out part of the primary winding section 16 displaces themagnetic centers of the primary and second- 3,315,197 Patented Apr. 18,1967 ary winding sections 16 and 15 so that the magnetic fluxdistribution is non-symmetrical and a substantial component of leakageflux exists in a radial direction and causes a large axial component offorce tending to displace the primary and secondary winding sections 16and 15 relative to each other. In accordance with the invention, anannular shield 34 of suitable conductive material such as copper isdisposed within the primary winding section 16 and preferablyimmediately adjacent the tap sections which cause the dissymmetry of themagnetic flux. As shown in the drawing, the shield 34 is disposed.between the first and second layers 20 and 21 of wire turns of primarywinding section 16. Shield 34 is of greater radial thickness than issuitable for mere electrostatic shielding and preferably is ofsuflicient thickness in a radial direction to cause the flow of eddycurrents of substantial magnitude when subjected to the radial componentof the leakage flux resulting from tapping out part of the primarywinding section 16. The conductive shield 34 extends for the entireaxial length of the transformer coil 14, and the generation of eddycurrents in shield 34 confines, or contains, the magnetic flux generatedby the winding beneath shield 34 for the entire length of the coil 14 sothat no appreciable component of the leakage flux flows in a radialdirection within the coil. The eddy currents link the magnetic fluxwhich causes them and are in such a direction as to oppose the change inthe magnetic field and tend to set up a magnetic field which opposes theincrease in flux density, thereby turning aside the leakage flux tendingto pass through shield 34 and suppressing the radial component of thecoil leakage flux. Consequently, the axial component of the mechanicalforce tending to displace primary and secondary winding sections 16 and15 relative to each other is materially reduced. Short circuit tests on50 kva. distribution transformers embodying the invention reveal thatthe ability of coil 14 to withstand short circuit without deformation isincreased from a magnitude of approximately 40 times rated currentwithout shield 34 to approximately fifty'five times rated current whenshield 34 is present. Further, by suppressing the leakage flux, shield34 improves the magnetic linkage between primary and secondary windingsections 16 and 15 and improves the reactance characteristics of thewindings.

The degree of correction, or suppression, of the leakage fluxdistribution is a function of the thickness of shield 34, and thethickness of shield 34 utilized in a specific winding is dependent uponthe degree of unbalance of leakage flux created by that winding and thedegree of correction desired. The thickness of shield 34 was varied inthe range of from 3 to 10 mils in experimental 50 kva. core typetransformers embodying the invention, and it was found that a shield 34having a thickness of .005 inch was adequate to substantially improvethe short circuit strength of the coils.

FIG. 2 schematically illustrates a cross section through a coil 40surrounding a magnetic core 11' of a shell type distribution transformerwherein low voltage winding sections 41 and 42 are disposed radiallyinward and radially outward respectively from a primary winding 44 withtubular insulating barriers 45 therebetween in what is termed alow-high-low arrangement. Low voltage winding sections 41 and 42 may beof conventional construction and comprise a plurality of conductor turnssuch as conductive strip or concentric layers of helical turns of wire43. Primary winding 44 may comprise nine concentric layers 46-54 of wireturns 55 with taps adjacent the middle of the winding connected to tapleads T3, T1 and T2 in the fourth layer 49 and to tap leads T6, T5 T4 inthe fifth layer 50. The tap leads are connected to stationary contacts(not shown) of a suitable tape changer in conventional manner. Tubularshields 56 and 57 of suitable conductive material such as copper aredisposed radially inward and radially outward from, but preferablyimmediately adjacent to, the layers 49 and 50 having the taps whichcause the non-symmetry of the magnetic flux distribution, and shields 56and 57 are preferably located between the third and fourth layers 48 and49 and between the fifth and sixth layers 50 and 51 respectively. Thegeneration of eddy currents in shields 56 and 57 corrects, orsuppresses, the radial component of the leakage flux generated by thehelical turns in the fourth and fifth layers 49 and 50, in the samemanner as explained for shield 34 of the embodiment of FIG. 1, andconfines the leakage flux principally between shields 56 and 57 so thatthe axial component of the mechanical force tending to displace theprimary winding 44 and the secondary winding sections 41 and 42 relativeto each other is materially reduced.

As shown in the drawing, the shield 34 of the embodiment of FIG. 1 (orthe shields 56 and 57 of the embodiment of FIG. 2) may be passive andelectrically connected to only one wire turn of the adjacent layer 21and separated by sheet insulation 29 from adjacent layers of wire turns,but in alternative embodiments the shield may constitute an activecurrent-carrying turn of the winding.

While only a few embodiments of the invention have been illustrated anddescribed, many modifications and variations thereof will be readilyapparent to those skilled in the art, and consequently it is intended inthe appended claims to cover all such modifications and variations whichfall within the true spirit and scope of the invention.

I claim:

1. In combination with an electrical transformer coil having concentricprimary and secondary windings each of which comprises a plurality oflayers of conductor turns, at least one layer of one of said windingsconstructed and connected to generate magnetic leakage flux having asubstantial radial component, an annular conductive shield disposedwithin said one winding adjacent said one layer and being of sufficientradial thickness to cause the circulation of eddy currents ofconsiderable magnitude in said shield due to said radial component ofsaid leakage flux, the radial thickness of said shield being in excessof that required for electrostatic shielding, whereby said shieldsuppresses leakage flux in a radial direction through said winding andimproves the ability of said coil to withstand short circuit strength.

2. In combination with an electrical transformer coil having concentricprimary and secondary windings one of which comprises a plurality oflayers of conductor turns and being provided with taps in one of saidlayers, whereby the magnetic centers of said primary and secondarywindings are displaced when part of said one winding is'tapped out, andannular conductive shield means disposed within said one windingadjacent said one layer having said taps and being of sulficient radialthickness to cause the circulation of eddy current of considerablemagnitude therein due to the radial component of the leakage flux fromsaid one winding, whereby said shield suppresses the radial component ofthe leakage flux in said coil and improves the ability of said coil towithstand short circuit current.

3. In the combination of claim 2 wherein said shield means includesannular conductive members disposed radially inward and radially outwardfrom said layer provided with taps.

4. In combination with an electrical transformer coil having concentricprimary and secondary windings one of which comprises a plurality ofconcentric layers of helical turns of wire and being provided with tapsin one of said layers, whereby the magnetic centers of said primary andsecondary windings are displaced when part of said one winding is tappedout, an annular conductive shield disposed within said one windingadjacent said one layer having said taps and being of sufiicient radialthickness to cause the circulation of eddy currents of considerablemagnitude therein due to the radial component of the leakage flux fromsaid one winding, whereby said shield suppresses leakage flux in aradial direction through said coil and improves the ability of said coilto withstand short circuit current.

References Cited by the Examiner UNITED STATES PATENTS 2,995,685 8/1961Lord 336-84 X 3,028,539 4/1962 Wright 33684 X FOREIGN PATENTS 235,394-8/1964 Austria.

LEWIS H. MYERS, Primary Examiner.

T. J. KOZMA, Assistant Examiner.

1. IN COMBINATION WITH AN ELECTRICAL TRANSFORMER COIL HAVING CONCENTRICPRIMARY AND SECONDARY WINDINGS EACH OF WHICH COMPRISES A PLURALITY OFLAYERS OF CONDUCTOR TURNS, AT LEAST ONE LAYER OF ONE OF SAID WINDINGSCONSTRUCTED AND CONNECTED TO GENERATE MAGNETIC LEAKAGE FLUX HAVING ASUBSTANTIAL RADIAL COMPONENT, AN ANNULAR CONDUCTIVE SHIELD DISPOSEDWITHIN SAID ONE WINDING ADJACENT SAID ONE LAYER AND BEING OF SUFFICIENTRADIAL THICKNESS TO CAUSE THE CIRCULATION OF EDDY CURRENTS OFCONSIDERABLE MAGNITUDE IN SAID SHIELD DUE TO SAID RADIAL COMPONENT OFSAID LEAKAGE FLUX, THE RADIAL THICKNESS OF SAID SHIELD BEING IN EXCESSOF THAT REQUIRED FOR ELECTROSTATIC SHIELDING, WHEREBY SAID SHIELDSUPPRESSES LEAKAGE FLUX IN A RADIAL DIRECTION THROUGH SAID WINDING ANDIMPROVES THE ABILITY OF SAID COIL TO WITHSTAND SHORT CIRCUIT STRENGTH.